Sixty-three years ago on Wednesday, the Supreme Court prohibited school segregation. In the South, Brown v. Board of Education was enforced slowly and fitfully for two decades; then progress ground to a halt. Nationwide, black students are now less likely to attend schools with whites than they were half a century ago. Was Brown a failure?

Not if we consider the boost it gave to a percolating civil rights movement. The progeny of Brown include desegregation of public accommodations and the mostly unhindered right of African Americans to compete for jobs, to vote, and to purchase or rent homes. Brown's greatest accomplishment was its enduring imprint on the national ethos: the idea of second-class citizenship for African Americans, indeed for any minority group, is now universally condemned as a violation of the Constitution and of American values. None of these transformations came easily, and none are complete, but none would have happened were it not for Brown.

Yet the decision could not accomplish its stated purpose. Today, nearly half of all black students attend majority black schools, with over 70 percent in high-poverty school districts. New York is the most segregated state: two-thirds of its black students attend schools that are less than 10 percent white. A growing number are "integrated" with low-income Hispanics and other recent immigrants, but still isolated from the mainstream.

Because schools remain segregated, we have little chance to substantially boost the achievement of black children, especially those from low-income families. Of course, some children will always surmount their disadvantages and excel. But when separate schools concentrate students who are in poorer health and more frequently absent, who may be homeless or in unstable housing, and whose parents are less-educated, achievement lags when teachers are overwhelmed by non-academic challenges.